The present invention relates generally to a stabilized sighting device suitable for use on all types of vehicles, while of particular interest on low speed aircraft such as helicopters.
Most stabilized sighting devices utilized in the past have included a core unit provided with a gyro and including the payload, which may include sensors and/or illuminators, mounted on a second unit for rotation about an axis (typically an elevation axis). The second unit is mounted on the vehicle for rotation about another axis (typically a bearing or azimuth), transversal to the first axis, by a motor or torquer controlled by a servoloop.
Often times, the device should have the ability to operate within a large range of positive and negative angular positions about the elevation axis. For instance, aiming sights for land vehicles, nose mast or fuselage mounted sights for helicopters should accept movements of considerable amplitude while maintaining a satisfactory accuracy and resolution in spite of movements and vibrations of the support. On the other hand, stabilization about the line of sight is generally unnecessary.
In such prior art devices where the motor for angularly locating the second (bearing) axis is controlled by a servo control loop, parasitic or perturbating inertial torques C.sub.e are applied about that axis, which constitutes an external axis: EQU C.sub.e =(I.sub.g +I.sub.v).omega. tan s (1)
where
I.sub.g is the moment of inertia of the first unit (bearing gimbal in general) alone, about the external axis (bearing axis); PA1 I.sub.v is the moment of inertia of the core about the sighting line; PA1 .omega. is the rotational acceleration about an axis perpendicular to the other two axes (elevational axis and bearing axis), i.e. an axis which is the roll axis at zero bearing; PA1 s is the elevation angle. PA1 I.sub.s is the inertia of the second unit (elevation gimbal in general) about the intermediate axis, such assembly being considered alone, to the exclusion of the sight unit, PA1 .omega. is the acceleration about the axis perpendicular to the elevation axis and "lateral" movement axis; PA1 1 is the "lateral" movement angle (angle between the line of sight and the axis perpendicular to the elevation axis and lateral movement axis).
Torque C becomes high when tan s becomes substantial, i.e. when the system is far away from the canonical position, in which the sighting line is orthogonal to the two rotation axes (bearing and elevational axes).
In view of the large values of torque C at high elevation angles, proper design of the system and determination of the stabilizing residuum imply that the actions to which the device will be subjected (particularly angular accelerations and energizing frequencies) are perfectly known, which is far from being always the case.
This problem is very different from those which arise in the construction of platforms stabilized along three axes and comprising gimbals mounted about pitch, roll and azimuth axes; the latter problems include flip of the roll gimbal when the amount of pitch exceeds 90.degree.. The use of two roll gimbals in cascade has been provided in this case, i.e. an inner gimbal having a small range of freedom and an outer gimbal equipped with a motor controlled by a synchrotransmitter carried by the internal gimbal (U.S. Pat. No. 3,188,870): the purpose of this arrangement is to facilitate flip. Applicants have found that a fundamentally different problem exists in respect of sighting devices in which the core has a much higher mass and inertia and consists in decreasing the incidence of the characteristics of the carrier on those of the device, by minimizing the multiplication coefficient of .omega. in the expression of the preponderant parasite torque in formula (1).
It is an object of the invention to provide a sighting device in which the tangent term of formula (1) is rendered small by maintaining the stabilizing system in a position close to the canonical position, whatever the elevation of the sighting line.
It is a more general object to provide an improved sighting device which has a high degree of accuracy when mounted on a vehicle.
According to the invention, a stabilized sighting device comprises a first gimbal unit mounted for rotation about an external axis, a drive motor for rotating said first gimbal unit about said external axis, a second gimbal unit mounted for rotation on said outer unit about an intermediate axis perpendicular to said external axis, and a sight unit provided with a gyroscope and mounted on aid second gimbal unit for rotation within a predetermined limited angular range about an inner axis located in a plane perpendicular to said intermediate axis. First and second motor means are provided for driving said second gimbal unit and sight unit about said intermediate and inner axes respectively. First and second servo loop circuits are connected to first and second outputs of said gyroscope for controlling said first and second motor means, respectively. Detector means are arranged to deliver a signal representative of the deviation of said first gimbal unit from the canonical position. A position reproducing loop controlled by said detector means actuates said drive motor.
In such a device, stabilization is obtained by controlling the motors of the angular movement servo loops about the intermediate and inner axes (elevational and "lateral" movement axes in most cases) directly from signals supplied by the gyro, whose spin axis is located parallel to the line of sight. A correction network will be provided in each loop for ensuring stability of the servocontrol.
On the other hand, the motor for moving the first gimbal unit angularly is controlled by a simple position copy system. The latter should fulfil one important condition: under all operating conditions, it must have a sufficiently low time constant for the angular deflection assumed by the sight unit about the inner axis to be small, typically within a range which does not exceed one to a few degrees. Thus, the sensitive axes of the gyroscope and the rotational axes about which rotation is controlled by the servo-control motors always remain practically co-linear and no network for compensating variations of the mechanical gain depending on the angular extent of movement about the intermediate axis (elevation angle) is necessary.
The position copying loop will comprise a compensation network for compensating variations of the mechanical gain responsive to angular amplitude of movement about the intermediate axis. But the implementation of such a compensation network raises much less problems than in a stabilization loop. One reason is that a stabilization loop must provide maximum values of stiffness and passband, which requires high gains and phase advance networks. To the contrary, the gain of the position copying system can be relatively low.
In the device of the invention, the critical or relevant parasitic torque is torque C.sub.s about the axis of the second unit (elevation axis in general) rather than torque C.sub.e. It is given by the formula: EQU C.sub.s =(I.sub.v +I.sub.s).omega.tg1 (2)
where
The arrangement which has just been described substantially decreases the effects of the torques which appear about the intermediary axis. To further improve the performance, it is desirable to reduce the amplitude of the torque. In an attempt to reduce I.sub.s, the assembly consisting of the sight and second unit will be advantageously given an inverted structure. Instead of forming the second unit as a fork straddling the sight unit, bulky parts of the latter will be located in parts placed on both sides of the second assembly and may thus have a low rotational inertia. Reduction of I.sub.s may be accompanied by an increase in I.sub.v, but the latter will generally be lower and the overall effect will be of advantage.
It will be appreciated that the favorable effect of the latter arrangement only exists because of implementation of the first one, which causes I.sub.s to appear in the parasite torque to be taken into consideration (formula 2).
The invention will be better understood from the following description of particular embodiments thereof, given by way of examples only.